A great amount of asbestos has been employed as heat-resistant reinforcement fibers in conventional friction material. However, asbestos-containing friction material producing wear powder by friction is going to be replaced by friction material containing other fiber materials. As substitute fiber materials, proposed and employed are aramid fibers (DUPONT TORAY KEVLER CO., LTD, Technical Document DTK-T01 91.2), acryl fibers (Japanese Unexamined Patent Publication Nos. 106133/1987 and 183950/1988) and carbon fibers (Japanese Unexamined Patent Publication Nos. 157671/1980 and 104378/1980, Japanese Examined Patent Publication No. 4455/1984). Carbon fiber is excellent in heat-resistant, wear-resistant and reinforcing properties ("Carbon Fiber", Mar. 1, 1986, KINDAI HENSYUSYA, pp557-567).
However, it is very difficult to obtain carbon fiber-containing friction material having a practical coefficient of friction and excellent properties in heat-resistance, wear-resistance and reinforcement, since carbon fibers has a drawback to decrease the coefficient of friction. When long carbon fibers are employed, friction material product having stable characteristics in strength, friction properties and wear-resistance can not be manufactured, since homogeneous mixing of base components can not be performed leading to formation of pills. In contrast, when short carbon fibers are employed, a great amount of particles or fibers of ceramics may be added so as not to decrease a coefficient of friction too much. In this case, friction material containing ceramics has a drawback to attack an element provided with the friction material. Further, aramid fibers, ceramics fibers and like reinforcing fibers must be blended in a great amount into the friction material because of insufficient reinforcement action of short carbon fibers.
Friction material is usually produced by the steps of mixing, molding and postcuring of raw material. Examples of mixing process of raw material for production of friction material are a dry process conducted by mixing constituents of raw material using mixers of V-type, planetary-type, cylinder-type, high speed fluid-type, rotating disc-type, pag mill-type, monoaxis rotary-type, etc., and a wet process conducted by adding a small amount of solution prepared by dissolving a binder in water or toluene, alcohols, methylethylketone and like organic solvents to a raw material constituents mixture prepared in a dry process, followed by kneading the mixture using mixers, such as wet mill, screw extruder, kneader, pag-mill, etc. until viscous and slurry product are obtained. However, it is very difficult to obtain a uniform mixture by the methods mentioned-above, when long carbon fibers are employed. Fiber components will be cut into short length due to strong shear force, if mixing is conducted in a severe condition for the purpose of obtaining a uniform mixture.
It is a major object of the invention to provide a method for producing friction material by effectively utilizing excellent properties of carbon fibers in heat-resistance, wear-resistance and reinforcement, and inhibiting a drawback of carbon fibers lowering coefficient of friction in some of use conditions.
It is another object of the invention to provide a method for enabling uniform mixing, when friction material containing fiber components are produced.